In recent years military aircraft, particularly those in the fighting class are outfitted with exhaust nozzles with enhanced capabilities. Inasmuch as the nozzles are subjected to extremely high temperatures, it is necessary to provide means for cooling the components in order to maintain their structural integrity. It is, of course, necessary that the use of cooling air be such as to not adversely impact the performance of the engine as by utilizing air that would otherwise be used for generating thrust. At the very least this use of this air should be held to a minimum and keep the performance deficit to a minimum. Such cooling systems also should be as light and as simple as possible for reasons of aircraft pay loads, engine operating performances and for aircraft and engine maintainability.
As one skilled in the aircraft and engine technology knows, it is abundantly important to cool the airframe and engine components that are disposed in the hot section adjacent the engine's exhaust nozzle. One method of cooling is by utilizing ejector nozzles to pump low energy ram air into the divergent section and film cool the hot nozzle components. While this system provides the lowest possible cooling air temperature it also provides the lowest possible cooling air pressure or cooling system outflow, therefore increasing maintenance and their attendant costs and inducing significant risk to the operation of the overall weapon system.
One solution to the problem noted in the above paragraph is utilizing interstage fan bleed air as a nozzle cooling air source. Interstage fan bleed air is available on turbo-fan engines that include multi-stages of fan rotors. A comparison of the temperature/pressure profile of the fan air at various stations in the multi-stages is demonstrated in FIG. 4 which is a plot of coolant temperature vs. coolant total pressure and the static pressure of the gas path adjacent the discharge of the coolant into the gas path. As noted, curve X shows that the coolant temperature and total pressure of the engine cycle air increases as it is bled closest to the fan discharge air. Obviously, this flow must be ducted directly to the exhaust nozzle in order to attain these values. This entails the necessity of ducting this air externally of the engine through external conduits. This would require that the basic engine configuration be changed in order to accommodate this cooling technique and would require large ducting to accommodate the air flow volume from the fan to the nozzle which would significantly add to the overall engine weight. And, additionally, this would have negative fan operability impacts.
It is also abundantly important to prevent gasses escaping from the aircraft engine from migrating into the nacelle and accumulating to the point where the gasses would combust and either cause fires or explosions. It is customary to purge the nacelle so as to avoid such occurrences. Current techniques for purging the nacelle is to locate a pump adjacent to the exhaust nozzle and pump the ram air that is contained in the nacelle through exhaust ports located at the tail end of the aircraft. Obviously, this entails pressurizing the stagnated gasses to assure that the proper exhaust flow is maintained throughout the operating envelope of the aircraft. The increase in pressure requires larger pumping apparatus resulting in heavier support structure that is necessary to attain the structural integrity of the engine parts which increases overall engine weight and hence, incurs an engine operating performance deficit.
One type of pump heretofore utilized for this purpose is an ejector pump that utilizes fan discharge air as the primary fluid and dumps the entrained nacelle air directly overboard. The use of fan air for this purpose and in this manner penalizes engine performance as the fan air would otherwise be used for generating thrust.
It is also important in this technology to pump nacelle cooling air at flow rates and pressures sufficient to be injected into the nozzle gaspath flow for sidewall cooling. Typically, one of two sources have heretofore been utilized for sidewall cooling. One of the sources is the fan air which has sufficiently high driving pressures but is at relatively high temperatures which results in poor cooling and as mentioned above comes at the expense of engine performance. The other source is the ambient air that is entrained in the nacelle which has sufficiently low temperatures that would provide efficient cooling, but its pressure is too low for it to penetrate into the engine's gas path and flow therein.
We have found that we can obtain sufficient cooling at the required coolant pressures by including boost pumps disposed locally relative to the exhaust nozzle. This is demonstrated in FIG. 4 where curve Y illustrates the boost air which is sufficiently cool and at sufficient pressure to be injected into the gas path. The boost pumps are ejector pumps that are powered by locally available fan air that is otherwise used for cooling purposes in the engine. In addition, by virtue of this invention this cooling system would provide sufficient nacelle ventilation which would eliminate the customary ventilation system that are typically provided in these types of aircraft.